Variable optical delay devices can be used in various optical devices and systems where the length of an optical path or the phase of an optical signal needs adjustment to achieve certain operating conditions. For example, the length of at least one of the two optical paths of a Michelson optical interferometer can be adjusted by using a variable optical delay device. A fiber-based time-domain optical coherent tomography (OCT) device is an imaging device based on the Michelson optical interferometer design and can use a fiber stretcher in a reference optical path to stretch a fiber loop as the variable optical delay device to change a phase difference between the reference beam in the reference optical path and a probe beam in a separate probe optical path coupled to a sample under measurement. As another example, a fiber laser may include a fiber stretcher as a variable optical delay device inside the laser resonator to adjust the cavity length of the laser resonator. As yet another example, an opto-electronic oscillator (OEO) that includes an electrically controllable optical modulator and at least one active opto-electronic feedback loop with an optical part and an electrical part interconnected by a photodetector can include a variable optical delay device in the optical part of the active opto-electronic feedback loop to achieve a desired phase matching condition and to tune the oscillation frequency. Examples of OEO designs can be found in, e.g., U.S. Pat. Nos. 5,723,856, 5,777,778, 5,929,430 and 6,567,436.